Electronic ballasts are widely used to power lighting circuits, including conventional fluorescent lamps, compact fluorescent lamps, and other fluorescent lighting components. Ballasts have been classified as magnetic ballasts and electronic ballasts, and with electronic ballasts various solid-state electronics are used to replace one or more magnetic components in the magnetic ballasts. The electronic ballasts can operate at a higher frequency, making fluorescent lamp operation more efficient, and more cost-effective.
Ballasts are intended to be constant current devices so that, once a fluorescent lamp is started and is illuminated, the lamp sees a constant supply of current regardless of the value of the voltage represented by the lamp. It will be understood that the discussion herein will refer to a single or dual lamp combination, but a ballast can be designed to drive more than two lamps at a time. However, for simplicity of the present discussion, a ballast will be described as driving a generic load, which may be a single lamp or a pair of lamps or other combination of lamps. For any given load, the voltage presented by the load to the ballast over time may change, either slowly or quickly. For example, as a lamp ages, the voltage presented by the lamp to the ballast increases while the ballast continues to supply the same amount of current. Consequently, the power dissipated through the lamp increases. This change may be gradual over a matter of weeks, months or years. In some situations, lamp aging might be accelerated with frequent cold starts of the lamp, particularly at relatively high voltages and using instant start ballasts that do not pre-heat the fluorescent lamp filament before full starting. Rapid start ballasts pre-heat the fluorescent lamp filament and then apply a high voltage to start the lamp.
Another condition that may lead to high power dissipation may include poor connections or corroded terminals on the lamp or a connector. As the quality of the connection deteriorates, the voltage presented to the ballast by the load increases, resulting in higher power dissipation. In some situations where a connection is poor or nonexistent, for example where a lamp comes loose from its socket or other connection and there is an air gap between terminals, the high voltage being produced by the ballast at the constant current may produce an arc through the air. The arc can produce high temperatures, melting wire insulation, wires, adjacent plastic components such as sockets, housings and other equipment or possibly start fires.
Some electronic ballasts have power factor correction while others do not. In some ballasts, passive power factor correction can be accomplished using a large inductor in series with the power line input. Active power factor correction can be applied in other ballasts using a boost circuit. In other ballasts having no power factor correction, the ballasts are generally considered to have normal power factor. General examples of ballasts are shown in U.S. Pat. No. 6,008,589, incorporated herein by reference.